Ductile iron is conventionally produced by adding nodularizing agents such as cerium or magnesium to molten iron that normally would produce a soft, weak grey iron casting. The addition of the alloying elements results in castings in which the carbon content (as graphite) is present in spheroidal form, which provides the casting with greater ductility than ordinary grey iron. Several types of matrix microstructures can be developed by alloying or heat treatment, such as pearlitic or ferritic matrices. Ductile iron may be defined with respect to a standard, such as American Society of Testing and Materials (“ASTM”) Standard A536, which specifies certain standard properties for ductile iron including: a tensile strength of at least 60 ksi, yield strength of at least 40 ksi and elongation of at least 18%, as well as methods for measuring those properties. In the industry, ductile iron meeting the ASTM A536 Standard is often referred to as “60-40-18” ductile iron.
Methods and alloys have been disclosed in the prior art for producing ductile cast iron with enhanced properties, including PCT International Patent Application Publication No. WO 1984/02924, which teaches a method for making a high-strength ferritic ductile iron by increasing the silicon, nickel and molybdenum contents of a relatively high carbon ductile iron composition to form an alloy consisting essentially of, by weight: silicon (Si) in a range of 3.9-6.0%; carbon (C) in a range of 3.0-3.5%; manganese (Mn) in a range of 0.1-0.3%; molybdenum (Mo) in a range of 0-0.35%; at least 1.25% nickel (Ni); no greater than 0.015% sulfur (S); and phosphorus (P) present at 0.06%; the remainder being iron (Fe). The casting produced is annealed to increase ferrite in the microstructure. While this composition has very high tensile strength and yield strength, this composition has insufficient elongation and toughness properties for the railcar applications contemplated by the present application.
U.S. Pat. No. 7,846,381, which is incorporated by reference, teaches high carbon, high silicon content cast iron formed with minimized nickel content, and without annealing, to obtain parts having high toughness. The resulting cast iron is described as ferritic, but may contain significant pearlite microstructure. The cold temperature toughness of the resulting product as measured by the Charpy V Notch test at −20° F. is only 6 ft. lb, which needs improvement. Thus, arriving at a desired combination of properties, which are sometimes competing in an iron alloy, is often elusive.